In general, an ion projection lithographic apparatus for the formation of an image of a structured mask, i.e. a mask provided with openings passing an ion beam, upon a substrate such as a silicon or other semiconductor wafer, will comprise an ion source generating the ion beam, a structured mask along the path of the beam, and an immersion lens between the mask and the wafer.
A projection lens, for example in the form of a Einzel lens unit, can be provided also in the path of the beam. Ion sources generally supply a mixture of ions and thus it is not uncommon for a helium ion source to also provide in addition to the desired helium ions, hydrogen ions which are not desired for the ion exposure and imaging on the substrate.
It is already known to exclude undesired ions, based upon their masses, originally present in an ion beam, from the beam before these ions reach the projection lens. In Miyauchi et al, Nucl. Instr. & Meth. in Physics Research B6, 183 (1985), an apparatus for maskless ion implantation in which, downstream of the ion source an ExB filter (Wien filter) can be provided, directly followed by a diaphragm. In the ExB filter, crossed electric and magnetic fields are generated, under the combined effect of which different ions of the same energy are subjected to different deflections, whereby, with appropriate setting of the electrical and magnetic fields, only ions of the desired mass will remain undeflected or so deflected as to pass through the diaphragm.
This apparatus is, however, only poorly suited for ion projection lithography, since the rays which pass out of the ExB filter are divergent, with an aperture angle of, for example 6. In order that the diaphragm directly downstream of the ExB filter can intercept the ions of undesired masks, these must be very sharply deflected by the ExB filter. This implies the need for very high electrical and magnetic field strengths at the Wien filter which are associated with strong inhomogenous boundary or edge fields contributing distortion to the image of a mask when the apparatus is used in ion projection lithography.
Another known apparatus is described in "Ion Implantation", Vol. 8, p. 446 (G. Dearnaley et al., North Holland Publ. Comp., 1973). In this arrangement proposed by M. Bernheim et al (4th Int. Conf. on Electron and Ion Beam Science and Technology, Los Angeles), downstream of the ion source, a magnet is provided which sharply deflects ions of different masses. Depending upon the adjustment of the magnetic field, only ions of a predetermined desired mass traverse the further elements of the imaging system. This method has, however, the serious drawback that the source cannot lie on the optical axis of the imaging lens system since the magnetic field deflects all ions, including those of desired mass.
Mention may also be made of the description of a Wien Filter in "Ion Projection Lithography" in Solid State Technology February 1986, page 119 to 125.
Japanese patent document 60-74249 appears to describe a maskless focusing ion beam system in which the ion beam is scanned over a substrate. This system is not described in conjunction with demagnification or 1:1 ion projection lithography through a mask foil which is structured, i.e. provided with openings and, while a diaphragm is described in this system, there appears to be no basis for assuming that this diaphragm, if used in conjunction with an ExB, would be provided other than directly adjacent the ExB filter as in the art previously discussed.